1. Field of the Invention
The present invention relates to a sheet processing apparatus, an image forming system, and a method of enhancing folding of a sheet bundle, and particularly to a sheet processing apparatus including a folding processing unit for folding a sheet-like recording medium such as paper, recording paper, or transfer paper (hereinafter referred to as “sheet” simply in this specification), an image forming system including the sheet processing apparatus, and a method of enhancing folding of a sheet bundle executed in the sheet processing apparatus.
2. Description of the Related Art
Some pieces of conventional post-processing apparatus used in combination with an image forming apparatus such as a copier produce saddle-stitched booklets in a manner that a sheet central part of a sheet or a plurality of sheets is bound and a central part of a sheet bundle is folded using a folding roller pair installed in parallel to the sheet folding direction. A technique is also known for sharpening the fold of the saddle-stitched booklet by enhancing folding of the sheet bundle with a roller moving along the back of the booklet.
In such a technique of enhancing folding, folding-enhancement rollers standing-by outside the booklet is put on the back (fold part) of the booklet (sheet bundle) for enhancing folding of the back of the booklet by the rollers; however, an end of the booklet may be damaged on this occasion. Further, when the rollers are moved from one end to the other end of the back of the booklet, the rollers run by the distance corresponding to the width of the paper forming the booklet; therefore, the twist is accumulated so that a crease or the like is easy to occur.
As a technique for dealing with this problem, for example, Japanese Laid-open Patent Publication No. 2009-1428 suggests a sheet post-processing apparatus capable of sharpening a fold with a sufficient amount of pressure without generating a curl or a crease at or near the fold part.
The invention of Japanese Laid-open Patent Publication No. 2009-1428 includes a saddle-stitching unit for binding a central part of a sheet bundle, a center-folding unit for forming a fold by folding the central part, first and second rollers for sharpening the fold by moving along a direction of the fold while pressing the sheet bundle conveyed from the center-folding unit with the fold of the sheet bundle interposed between the rollers, and a driving unit for moving the first and second rollers in a direction along the fold from a standby position away from an end of the sheet bundle. The first roller and the second roller are separated from each other at the standby position, and approach to each other in a region of the fold of the sheet bundle to have the fold interposed therebetween.
That is, Japanese Laid-open Patent Publication No. 2009-1428 discloses a structure in which, when the back of the sheet bundle (fold part) is folding-enhanced by the roller pair, a nip is released at a position where the sheet bundle does not present and the sheet bundle is nipped at a position where the sheet bundle presents, thereby reducing the damage on the end of the paper bundle (sheet bundle).
However, since the sheet bundle is folding-enhanced from one end to the other end of the sheet bundle without pausing, the twist that would cause the crease or the like are accumulated, which may result in the generation of the curl or crease at or near the fold part.
In view of this, there is a need to suppress the curl or crease generated at or near the fold part due to the accumulation of the twist without damaging the end of the sheet bundle when the sheet bundle is folding-enhanced.